Water in arid places: Reticular Chemistry, MOFs, COFs, weaving and pathways to practical nanotechnology

Overview

I recently noticed that New Scientist made mention of a particular application of some particularly interesting research directed by Omar Yaghi, an American-Jordanian chemist at UC Berkeley. This work is notable for a number of reasons. Perhaps the best introduction to it is this video, here, which was recorded at Stockholm University May 9-10, 2019 earlier this year:

Notes from the talk

In the video, a few notable things are mentioned:

  • Up until the 1990s – indeed, 1993:
    • polymer chemistry (1d chains) was state of the art
    • 0d (organic chemistry) was well understood
    • 2d, 3d chemistry was not well understood
  • The basic game of chemistry is to create compounds that aid synthesis of other compounds
    • In order to have reproducible results you need to crystallise your catalytic compounds
    • But the crystallisation problem was not solved
    • A key problem with crystallisation is understanding how to deal with compounds with bonds that are stronger than Van de Waals forces, Hydrogen bonds, and M-donor bonds in coordination networks

In the late 1990s, Omar made progress with reticular chemistry with MOF (metal organic frameworks) that solved the crystallisation problem. These used M-charged linker bonds.

Later still, in the 2000-2010 decade, Omar made progress with Covalent organic framework using covalent bonds.

Together, these two forms of stronger bonds form the basis of what he calls Reticular Chemistry.

Generalisations of COFs include molecular weaving (~2016). Current research involves using a mixture of heterogeneity in crystalline backbone (rather like DNA) to mimic proteins (i.e. organic molecular machines) in a much wider variety of reaction conditions. i.e., nanotechnology.

But the rest of his talk didn’t really focus on his work from 2000+; rather, it focused on applications of the pure research on MOFs dating from the 90s and its gearing for practical applications (a company based on these applications, Water Harvesting Inc. (http://www.wahainc.com/) is due to launch in October this year).

Basically his point was that with MOFs you can build a massive massive amount of combinatorial possibilities of catalytic compounds that can do various things, depending on:

  • the choice of metal
  • the choice of organic molecule
  • the choice of geometry

These form a ‘periodic table’ of reticular chemistry of tremendous combinatorial complexity. “[If you can imagine it, you can build it]” paraphrasing Omar in his talk.

Possibilities of applications of such combinatorial choices include:

  • hydrogen storage (for hydrogen powered fuel cells)
  • methane storage (up to 3 times that of just storing the methane without the MOF)
  • carbon dioxide sequestration and conversion into methanol fuel (work in progress, currently in the lab, but with obvious applications to combating global warming while producing a useful product to boot)
  • water generation in arid air

He spoke a bit about water generation. There are apparently 6 septillion litres of water in the air at any one time – as much as is present in freshwater lakes and streams. Apparently a lot of current desalination technologies rely on high humidity (~65%) and also for the air to be cooled to ~1.5 degrees celsius for the Carnot cycle / Dew point to be reached. So 1) very energy inefficient and 2) not typical of desert conditions, where humidity might be between 5% and 25-30% at most during the night.

His team ultimately created a device that served as the desiccant, i.e. with the MOF as the desiccant. It required no power – just sunlight. And it generated (first prototype with zirconium) about 0.2L of water per day per kg of MOF. The second prototype (since zirconium is about $150 USD / kg) used aluminium, and produced about 1L of water per kg per day.

Bottom line

Water Harvesting Inc. is launching passive water extractors from low humidity air in October; they (wahainc) are looking to commercialise this technology, and release very soon. A tremendous success story and significant for all sorts of reasons; if nothing else because:

  • This rudimentary application of basic nanotechnology has the potential to solve water problems in water stressed regions
  • This technique promises fresh water where potable water is otherwise not present
  • This technique also provides a potential route to carbon dioxide sequestration and mitigation of global warming

More excitingly, the future theoretical direction of this research seems like a nice natural pathway to look into designing customisable and specialised molecular machines for performing very specific tasks. Not programmable / adaptive machines by any means – that would be assembler level control – but one could certainly potentially imagine the ability to program heterogeneity into a crystalline MOF or COF to perform a specialised operation at one further level of abstraction, so that one would be in essence building reaction vessels at runtime in order to build something more complicated at molecular level. That would be an assembler, or a pathway to creating assembler prototypes.

References

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